1. Field of the Invention
The invention relates to a sidestream gas sampling system having a gas sampling circuit that is closed to motive components and/or analysis components of the gas sampling system.
2. Description of the Related Art
Gas analysis apparatus are useful for many applications, including medical uses. Examples of medical uses of gas analysis apparatus include capnography and oxygraphy (or oxigraphy). Capnography refers to the measurement of carbon dioxide (CO2) in a patient's breath. Oxygraphy refers to the measurement of oxygen (O2) in patient's breath. These measurements may provide useful information regarding the patient's health. Additionally, other gases in the breath may be monitored for medical purposes including therapeutic gases (such as nitric oxide), anesthetic agents (such halothane) and trace gases used for diagnostic purposes (such as nitric oxide and carbon monoxide, which are typically present in the parts per million (ppm) or parts per billion (ppb) levels). Trace substances in the breath may reflect the concentration of the substance in the blood of the patient.
Information provided by capnography is used, for example, to detect incidents that may occur during medical procedures, particularly the detection of general anesthesia incidents. Additionally, information provided by capnography is used during clinical events such as, for example, cardiac or respiratory arrest. In fact, capnography can provide information in real-time regarding the effectiveness of resuscitative efforts during cardiac arrest. As such, capnography has multiple medical uses during medical procedures, during prolonged monitoring of patient vital signs, during clinical events, and other medical uses.
Oxigraphy also has multiple medical uses. Oxygraphy measures the approximate concentration of oxygen in the vital organs on a breath-by-breath basis and can quickly detect imminent hypoxemia due to decreasing alveolar oxygen concentration. For example, during hypoventilation, end tidal oxygen concentration changes more rapidly than does end tidal carbon dioxide. During the same conditions, pulse oximetry takes considerably longer to respond.
Oxygraphy has also been shown to be effective in diagnosing hypoglycemic or septic shock, air embolism, hyperthermia, excessive PEEP, CPPR efficacy, and even cardiac arrest. During anesthesia, oxygraphy is useful in providing a routine monitor of preoxygenation (denitrogenation). It especially contributes to patient safety by detecting human errors, equipment failures, and disconnections. A paper by M. Weingarten entitled “Respiratory monitoring of carbon dioxide and oxygen: a ten-year perspective” (J. Clin. Monit. Jul. 6, 1990(3):217-25), which is hereby incorporated by reference herein in its entirety, highlights some of the applications of monitoring carbon dioxide and oxygen in the patient's breath.
There are typically two principal methods of sampling gas for gas analysis. The first is mainstream gas analysis (i.e., non-diverting gas analysis), which measures the concentration of a gas or gases (e.g., a patient's breath) at a sample site located within the respiratory gas stream. For example, a patient being treated with a respirator has a patient circuit extending from his or her oral cavity that communicates a flow of gas between the respirator and the patient. A mainstream gas analyzer measures the concentration of a gas or gases within the patient circuit.
The other principal method of sampling gas for gas analysis is sidestream gas analysis (i.e., diverting gas analysis). Sidestream gas analysis transports gas away from the sample site and measures the concentration of gas or gases in the transported sample at a remote location. For example, if a patient is being treated by a respirator, a diverter/adaptor is coupled to the patient circuit to remove a portion of the gas traveling through the patient circuit and transport that gas a certain distance away from the patient to a gas measurement device. The concentration of the gas or gases within the sample may then be measured by the gas measurement device and the gas sample may then be disposed of.
The nature of sidestream gas analyzers imposes certain requirements as to their component parts. Similar to mainstream gas analyzers, sidestream gas analyzers must include analysis components (e.g., spectroscopic) and a sample chamber. Gas is transported into the sample chamber and the gas concentration is measured using the analysis equipment. In many instances, spectroscopic analysis equipment is employed, which uses the infrared absorption of gases of interest to measure the concentration of these gases in the sample (e.g., CO2, O2, anesthetic agents, etc.).
Sidestream gas analyzers may also include an apparatus such as, for example, a pump to create negative pressure that draws the gas sample from the sample site. Additionally, sidestream gas analyzers may include pressure measurement devices to measure the pressure within the sampling apparatus. Information regarding the pressure within the sampling apparatus may be useful to measure and/or correct for the effects that pressure within the sampling apparatus has on the absorption of infrared radiation by gases in the sample. Pressure measurements may also be useful for recording and/or compensating for pressure drops or other fluctuations within the sampling tubing and other components of the sampling apparatus. Other uses for pressure measurements may also exist.
Furthermore, sidestream gas analyzers may include flow measurement devices to measure the flow of gas through the sampling apparatus. Information regarding the flow within the sampling apparatus may be used to adjust or regulate the pump to maintain a constant flow rate within the sampling apparatus under a variety of load conditions. A constant flow rate may be desirable when measuring gas concentration over an extended period of time, as it simplifies the compensations required. Less constant flow within a sampling apparatus may also be used, but requires additional compensation calculations. Flow rate information may also be useful for other purposes.
In some environments, the sampled gas may be routed back into the patient circuit after analysis by a sidestream gas analyzer. This is sometimes done, for example, in situations where an expensive anesthetic is used on the patient that can be conserved by reintroducing it into the patient circuit. Additionally, the sampled gas of the patient often contains contaminants (e.g., mucus, blood, medications, or other materials). Routing these materials back into the patient circuit is sometimes considered a viable option for placement of an analyzed sample containing these contaminants.
However, when routing analyzed gas back into a patient circuit using a typical sidestream gas analyzer, care should be taken to prevent contamination of the internal capnometer parts. In a typical sidestream gas analyzer, sampled gas makes contact with pump parts, pressure measurement parts, flow measurement parts, tubing, water traps, and/or other parts of the capnometer. If the sidestream gas analyzer is to be used on multiple patients, the patients may be at risk from cross-contamination if the sampled gas is routed back into the patient circuit via a connection to an exhaust port or purging of the sampling lines to keep them clear. Because many of these parts are relatively complex (e.g., pumps, pressure transducers, flowmeters, spectroscopic equipment, or other complex parts), the cost of replacing them due to degradation in performance or occlusion from excessive exposure to contaminants may be large.